Create inverse gradient opacity

Dear members, I just start learning Adobe Illustrator. I want to create this blue-ish effect on the map you see below. The layers I am using is Slovenia (layer_1) and the countries surrounding Slovenia as a sepperate layer (layer_2). I don't know if opacity is the correct method to use, so I apologize for the subject's title. Also, I don't know if layer is the right terminology for the two files I am using (Slovenia and the other countries). Any ideas on how can I achieve this effect? Many thanks.